The present invention relates to conversion from color image information to colorant signals of, for instance, cyan, magenta, yellow and black in digital color copiers, digital color printers, color facsimile machines, etc.
Digital color copiers etc. using colorants of cyan (C), magenta (M), yellow (Y) and black (B) are associated with a problem that in outputting a high-density black or dark gray image, black reproduced with three colorants of C, M and Y (hereinafter called "process black") is not perfectly black, i.e., slightly colored. Further, since the process black is produced as a superposition of the three colorants of C, M and Y, a misregistration of the three colorants causes a color blur around a black portion, which is a problem in printing black characters.
It is a common procedure to avoid the above problem that black is reproduced with a single colorant K (hereinafter called "single black") instead of process black. This procedure of using single black instead of process black to thereby decrease amounts of the colorants C, M and Y is called gray component replacement (GCR) or under color removal (UCR).
FIGS. 4(a) and 4(b) illustrate a conventional GCR technique. FIG. 4(a) shows colorant signals of C, M and Y for reproducing a certain color on a document in a digital color copier. As shown in FIG. 4(b), respective portions of the colorant amounts of C, M and Y, that is, portions associated with the minimum of the colorant amounts of C, M and Y, are replaced with a colorant amount of K. Those respective portions are subtracted from the colorant signals of C, M and Y.
However, since the above conventional technique is not properly supported by colorimetric theories, a desired color is not obtained and the chroma is much reduced. Various improvements have been proposed to avoid this problem.
Among those is a technique disclosed by Sayanagi and Tamune in "Considerations (I) on Black Addition in Printing," Proceedings of First Color Engineering Conference, 1-7, pp. 33-36, 1984. In this under color addition (UCA) technique, C, M and Y are added to prevent the chroma reduction.
In Japanese Patent Application Unexamined Publication No. Sho. 64-45642, color image signals are represented on the CIE 1976 L*a*b* uniform color space and colorant signals of C, M, Y and K are set so that their variations are approximately proportional to variations of chromaticity values on the L*a*b* uniform color space. With this technique, the colorant signals of C, M, Y and K can be determined by simple calculations.
According to the conventional techniques described above, the colorant signals of C, M, Y and K can be determined with a relatively high accuracy when the ratio of replacing the process black with the single black (hereinafter called "GCR ratio") is fixed. However, for the following reasons, the GCR ratio needs to be changed, for instance, in digital color copiers. In copying a text, it is desired that black characters be reproduced without causing color blurs by achromatic printing (GCR=100%) in which process black components are fully replaced by single black. On the other hand, in copying a natural image, the GCR ratio should be small because a large GCR ratio will cause a rough image.
Since the relationship between the three input image signals and the four colorant signals is not clearly defined, the types of conversion parameters to be changed and their variations are complex and unclear. Therefore, the conversion to the colorant signals of C, M, Y and K cannot be performed with a sufficient accuracy, to cause a problem that the color of output images varies with a variation of the GCR ratio.